In telecommunication systems, customer service is generally provided over transmission cables comprising numerous conductors contained within a protective shield which includes a low resistance, continuous metallic layer. From time to time, a resistive shunt fault occurs that may trace to manufacturing defects or, more usually, to damage during cable usage and the presence of moisture in the cable. Such faults, because of their service affecting nature, must be located and repaired.
Conventionally, the first step in locating such a fault, which resistively couples two or more conductors, is that of making reflectometer or bridge-type measurements on the faulted conductors, typically from both ends of the cable section affected by the fault. The objective is to provide an approximate distance to the fault, that is, the fault is "localized." The fault may then be precisely located with portable test equipment that utilizes tracing tones.
High resistance shunt faults have proven to be particularly difficult to localize because they cause only a minor perturbation on the transmission characteristics of the medium and, therefore, sensitive test apparatus is required. Moreover, conventional bridge measurements rely on the availability of a nonfaulted wire of known resistance or a good pair. When all other pairs in the cable are working pairs or faulted (e.g., pulp cable saturated with water), bridge measurements may be impractical.
The article entitled "Locating Cable Faults," by C. A. Maloney, IEEE Transactions on Industry Applications, July/August 1973, presents a discussion of representative prior art techniques utilized to locate resistive shunt faults. The conventional bridge techniques, and usual variations thereon, are discussed on pages 385 and 386. Time domain reflectometer (TDR) techniques are presented on pages 387 and 388.
With a bridge technique, one conductor from the faulted pair of conductors is connected so that the conductor sections on either side of the fault become legs of the bridge. The fault is generally placed in series with the bridge supply voltage. Another conductor (or conductors) is (are) required to connect the far end of the faulted conductor back to the bridge's instrumentation. The return wire(s) must be good for accurate measurements and some measurements require the good wire to be the same length and gauge as the faulted wire.
A reflectometer measures the time required for a pulse to be reflected from the fault. High resistance faults reflect very little energy and it is difficult to distinguish reflections from other discontinuities such as gauge changes and bridged taps.